In general in a microwave oven provided with a microprocessor controlling the operation of the magnetron, the food treatment (thawing, thawing plus heating, or thawing plus cooking) can be preplanned by the user by setting the food weight, the food type and the required type of treatment. These data enable the microprocessor to control the magnetron operation at the power and for the time required to obtain the desired treatment.
To be able to operate automatically on the magnetron so as to fix its operating power and control the food treatment without the weight of the food having to be keyed in manually by the user, this weight is determined for example by placing the food on a support to which a weight measurement device or more simply balance is connected. The balance thus enables the food weight to be automatically fed to the microprocessor by merely placing the food in the oven.
Such a system is much used. It has however many drawbacks including the high cost of the components and the fact that the weighing device is a very sensitive and delicate mechanism which means that it can be easily damaged during the insertion of the food into the oven and/or during the oven operation.
Alternatively, to automatically control the treatment of a food in a microwave oven (for example its thawing), weight sensors are provided in the oven to measure the variation in the weight of the food as said operation proceeds. These sensors are connected to the microprocessor which, on the basis of a preset program and the data obtained by the sensor, halts the operation of the microwave generator and thus the thawing when the weight of the food has reached a predetermined value.
As an alternative to the aforesaid there are microwave ovens provided with infrared sensors which measure the surface temperature of the food. These sensors, connected to a microprocessor, cause the control member to act on the microwave generator and halt its operation when the food surface temperature has reached a predetermined value.
In both cases a microwave oven as described is costly and laborious to construct. In addition the data obtained by the sensors do not always reflect the true thawing level attained by the food, particularly with regard to its interior.
To determine the weight of a food contained in the microwave oven and on this basis set the operation of the magnetron and control the treatment of the food, it is known to measure the quantity of water contained in it when the food is frozen or is in any event at low temperature. This forms the subject of preceding patent applications in the name of the present applicant, which claim the use of bodies constructed of microwave-sensitive material disposed below the food in appropriate cavities provided in the oven casing. These bodies absorb the microwaves generated by the magnetron to a different extent depending on the free water contained in the food and on the temperature of this latter. The greater the quantity of free water present in the food the smaller the amount of microwaves which reach the body lying below it, and the less the body heats up.
As the percentage of water in foods is proportional to the weight of the food itself for each food type, the food weight can be determined by indirectly measuring this quantity of free water. To obtain this measurement, the bodies are associated with probes which measure their temperature and feed the measured data to the usual microprocessor which by knowing the quantity of free water present in the food, can use this information to obtain its weight, set the operation of the magnetron and control the treatment of the food.
In this respect it has been surprisingly found that the heating curve of such bodies of microwave-sensitive material becomes, within a small time period from application of the microwaves, a straight line for temperature/time, the slope of which varies according to the food weight.
It is apparent that if at the end of this time period T a measurement is made either of the slope of this straight line or the point of intersection with the temperature axis of a straight line parallel to the time axis and passing through the point which the temperature/time line has reached after the time period t, the microprocessor receives information corresponding to the food weight and, on the basis of this and taking into account the information fed in by the user regarding the food type and the required treatment, is able to fix the operating power of the magnetron and the duration and mode of treatment.
Although such a solution enables satisfactory results to be obtained, it is of complicated construction particularly with regard to the formation of the seats for the microwave-sensitive bodies or elements and the seats for the probes. In addition, as the heating of the elements depends on the degree to which the foods positioned above them are transparent to the microwaves, any mistake in positioning the foods in the cooking chamber falsifies the data received by the microprocessor and thus falsifies its action on the magnetron and its control over the food treatment. In addition, once the microwave-sensitive elements have become hot it is not possible to reuse them for determining the weight of another food to be treated until they have cooled down, otherwise the weight determination is inaccurate.